Many cell lines are very sensitive to fluid-mechanical stresses such as shear forces. Generally, animal cells, insect and plant cells are difficult to culture on a large scale due to shear sensitivity. Bioreactors for shear sensitive cells must provide gentle agitation so as to minimize shear, while still providing sufficient mixing and aeration for growth. The balance is difficult to maintain.
Conventional airlift bioreactors typically include a concentrically placed draft tube within the bioreactor vessel. Air is introduced at the base of the reactor, creating a density difference in the liquid medium. The rising bubbles provide oxygen for growth and circulate the cells and liquid medium by airlift. Such bioreactors tend to cause strong fluid shear force detrimental to growth and productivity for shear sensitive cells. The bubbles may coalesce into larger bubbles as they rise, and upon contacting the surface, the bursting bubbles create extreme shear stress on the cells (bubble shear) leading to metabolic stress or cell destruction.
Classical stirred tank bioreactors provide aeration through a sparger, pipe or perforated ring at the bottom of the reactor vessel. Agitation is accomplished by impellers such as flat plates, helical blades or a screw type auger mounted on a central rotating shaft. Such means of agitation and aeration generally cause turbulent flow characteristics resulting in fluid-mechanical shear stress to shear sensitive cells.
Several bioreactors have been designed specifically for shear sensitive cells. For instance, U.S. Pat. No. 4,906,577 issued May 6, 1990 to Armstrong et al. discloses a bioreactor having a lower stirred cell culture tank and an upper compensation chamber so as to operate with zero head space. A gas exchange tube is located in the culture tank to cause an outer downflow zone and a central upflow zone. A screw type auger stirrer is centrally located in the tank.
U.S. Pat. No. 4,649,117 issued Mar. 10, 1987 to Familletti et al. provides a bioreactor having two chambers, an upper wider chamber and a lower, smaller diameter chamber connected by inwardly sloping side walls. Agitation is accomplished by introducing a gently flowing gas stream centrally at the base of the lower chamber.
The above reactors suffer several disadvantages, including complexity of design and/or undesirable levels of cell damage due to fluid-mechanical shear or bubble bursting.
Several inclined bioreactor designs have been proposed in the past. Japanese Patent 58-134989 discloses a rotatable cylindrical culture tank. The tank is rotated about its horizontal axis and gas is sparged through a horizontal pipe at the bottom of the tank. The tank may be inclined at an angle less than 45.degree. from the horizontal. An angle greater than 45.degree. is stated to cause settling problems which necessitate rotation at a too high of a rate.
Japanese Patent 62-44173 discloses a similar rotatable cylindrical bioreactor. The axis is inclined at 5-55 degrees from the horizontal. The reactor is rotated along its axis to cause a gradient in the distribution of adhesion dependent and floating type cells.
U.S. Pat. No. 5,057,429 issued Oct. 15, 1991 to Watanabe et al. discloses a cell culture apparatus wherein the cells and culture media are contained in a semipermeable bag which is rotated or shaken at various angles.
Agitation in the above reactors is achieved by partial rotation, shaking or rotation tumbling, all of which cause unacceptable levels of mechanical shear and bubble shear, as discussed hereinabove. Furthermore, the bioreactor design is complicated by the need to provide complex means for rotation together with seals for gas inlets and shafts into a rotating vessel.